This invention relates to a piezoelectric transformer, and more particularly to a small size type thickness mode vibration piezoelectric transformer using piezoelectric ceramics.
In recent years, for the purpose of miniaturizing power supply unit for communication and computers, the usage of the switching type power supply unit operable in a high frequency range is widely studied and being developed rapidly. For miniaturization of the switching type power supply, many prior arts attempt to use electromagnetic transformers. However, the use of higher frequency is required in many field. In view of this high frequency application, the electromagnetic transformer has many disadvantages involving magnetic material of the electromagnetic transformer, such as sharp increase in hysteresis loss, eddy-current loss and conductor skin-effect loss. Those losses limit the upper limit of practical frequency range to 500 kHz at most.
On the other hand, piezoelectric ceramic transformers (hereinafter, referred to as piezoelectric transformer unless otherwise specified) are operated in resonance, and hence compared with a general electromagnetic transformers, they have many advantages, for example, (a) higher energy density at the same frequency may permit miniaturization; (b) improvement for making incombustible; and (c) no noise due to electromagnetic induction.
A typical conventional piezoelectric transformer known as Rosen type transformer is illustrated in Electronics Letters Mar. 31, 1988, Vol. 24, No. 7, pages 444 to 445. Referring to FIG. 1, description will be given for the case of obtaining output of high electric voltage using the Rosen type transformer. A vibrator portion 1 or low impedance vibrator portion is interposed between electrodes 3 and 4, and polarized in the direction of thickness indicated by an arrow. A generator portion 2 having a high impedance is provided with an electrode 5 at one end face and polarized in the direction of length indicated by another arrow shown. The piezoelectric transformer is operated by applying alternating voltage to external terminals 6 and 7 connected to the drive electrodes 3 and 4, respectively. A longitudinal vibration is excited in transverse effect 31 mode according to an electromechanical coupling coefficient K.sub.31, and in turn the generator portion 2 is excited in a longitudinal effect longitudinal vibration mode (33 mode) according to an electromechanical coupling coefficient k.sub.33. As the result, high voltage output is obtained between the terminal 7 and another terminal 8 connected to the electrode 5. On the other hand, for obtaining output of step-down voltage, as appreciated, the high impedance portion undergoing longitudinal effect may be used as the input and the low impedance subjected to transverse effect as the output. To the conventional piezoelectric transformers like this can be applied up to 200 kHz. Further the Rosen type piezoelectric transformer has been found disadvantageous in that any coupling coefficient other than k.sub.31 which is a very small transverse effect longitudinal vibration mode can not be applied for it, resulting in obtaining a small band width only.
Piezoelectric transformer, different from electromagnetic transformer, has a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and thickness of material as of piezoelectric ceramics or electrodes, and accordingly may vary with shrinkage and others when manufactured. The resonant frequency however is not always identical with the drive frequency of the external circuit.
With increasing the mechanical quality factor of the piezoelectric transformer, the power transmission efficiency can become higher. After sintering followed by no further processing, it was found to have bad parallel precision and planeness, and give results of a small mechanical quality factor and lower power transmission efficiency.